The invention concerns a surface pattern of the kind set forth in the classifying portion of claim 1.
Such surface patterns have a microscopically fine relief structure and are suitable as optical security elements for enhancing the degree of security in terms of forgery of value-bearing papers or securities and bonds, passes, means of payment and similar articles.
A surface pattern as set forth in the classifying portion of claim 1 is known from European patent application EP 247 471. The surface pattern has three surface portions with an optically effective diffraction structure. Those structures diffract visible light according to the wavelength thereof at different diffraction angles. The profile height of the grooves or furrows of the three structures is constant in each surface portion but it is fixed differently in each surface portion in such a way that for a given observer, the first structure diffracts blue light, the second structure green light and the third structure red light, with respective vanishing or minimum diffraction efficiency. When the surface pattern is tilted about an axis which is parallel to the furrows of the structures, in accordance with the teaching of the application therefore the first surface portion should appear dark at a first viewing angle, the second surface portion at a second viewing angle and the third surface portion at a third viewing angle, that is to say, a dark spot should abruptly change in position as the surface pattern is continually tilted. When the surface pattern is tilted however the angle of incidence of the light impinging thereon also changes and therewith also the optical profile height of the structures. In that situation the condition that the predetermined spectral color is not diffracted or is diffracted only with a very low level of efficiency is no longer satisfied or it is satisfied only in exceptional cases. If the profile height is selected to correspond to a minimum in regard to the level of diffraction intensity for a predetermined spectral color the surface portions are also usually only relatively weakly visible in the other spectral colors. If the illuminating light source is also wide-spread, which is the case with diffuse daylight in the open air or under a neon tube, then the light is no longer incident from a single defined direction of incidence but impinges on the surface pattern from many directions of incidence. Therefore the surface pattern simultaneously diffracts light of various colors into the eye of the observer. That effect is further increased by the roughness of the substrate. In particular paper substrates have a relatively rough surface. If the observer therefore views the surface pattern for example from a direction into which only green light is diffracted upon illumination with a punctiform light source and when the substrate has a smooth surface, then red and blue light are also diffracted into that direction when the light source is spread and the substrate has a rough surface. The desired effect is therefore greatly reduced or is no longer perceptible at all.
EP-712 012 A discloses a grating with a number of lines of more than 2000 lines per mm, which is effective as a color filter only in the zero diffraction order. In the direction of the zero diffraction order the light which is reflected at the structures of D2 is no longer white but colored as, for certain depths in respect of the impressed structure, one color of the spectrum or the other is entirely extinguished. In particular, in the case of a grating with a rectangular structure, color saturation and color purity can additionally be controlled by varying the peak-to-valley ratio. Tilting the grating structure about an axis in parallel relationship to the grating lines does not alter the color.
The object of the present invention is to propose a surface pattern having diffraction structures which generate optically variable effects which are clearly visible under virtually any illumination conditions and which can therefore be easily checked by the person in the street.